This invention relates to the accurate and sensitive measurement of small neutron fluxes existing on the exterior of a nuclear reactor during reactor start up. More specifically, the invention relates to a scintillation type neutron detector having maximum resistance to detector sensitivity degradation in high radiation flux environments.
In order to accurately monitor and control the start up of the nuclear reactor, some means must be devised to monitor the power level achieved by the reactor during start up. Since reactor power is always proportional to neutron flux levels, rapidly responding neutron detectors are desirable for this application. Detectors located outside the pressure vessel are preferred since they are not subject to limitations of space existing within the core, and because less hostile environmental conditions (radiation levels four magnitudes lower and ambients at least several hundred degrees cooler) assure longer instrument lifetimes with adequate accuracy and reliability. The prior art has heretofore commonly used B.sup.10 and BF.sub.3 ex-core start up detectors located in detector wells formed in the biological shield adjacent to the reactor pressure vessel. These ex-core start up detectors have exhibited an unacceptable failure rate primarily due to high gamma radiation levels and high neutron radiation levels which can vary by as much as 10 to 12 orders of magnitude between reactor start up and peak power. The power levels themselves extend from zero to perhaps twice the full rated range.
With current technology, no single instrument channel can provide satisfactory control over such an extensive range. Therefore, the usual approach is to divide the complete range of measurement into three separate, smaller ones, with a certain amount of overlap between adjacent ranges. At the bottom of the scale is the "source range" of control. With a reactor in the quiescent state, before being started, the rate of spontaneous fission among the uranium atoms is barely perceptible if no external neutron stimulus is present. If the configuration of fuel assemblies plus control rods is such that a single entering neutron could trigger a rapid chain reaction, anticipation and prevention of uncontrolled onset of criticality would be very difficult. To avoid this possibility, a neutron source is installed in the reactor and kept there at all times; this assures a measurable count, even when the reactor is in the safe shutdown condition.
Over the entire "source range," neutron production rates are so small that they are measured in terms of individual neutron pulses, and meter display is in counts per second. The range covers five to six decades of neutron pupulation, or reactor power. At the low end, safety dictates measured count rates of one to ten counts per second. Thus, an extremely sensitive neutron detector is desirable. In addition, the upper limit of the range depends on the ability of a detector and its circuitry to discriminate discrete neutron pulses without saturating.
Within a short time following a reactor shutdown, gamma ray flux levels due to prior operation can be sizeable, even though neutron population may be quite low. Therefore, care must be exercised in discriminating between counts resulting from actual neutron population and counts resulting from a phenomenon called "gamma pile up" in which two or more gamma rays activate the detector at the same time with the result that a pulse having a magnitude equal to the sum of a number of gamma pulses is generated.
Typically, gamma pulses are discriminated from neutron pulses by pulse size. The neutron count rate may be masked, however, by this phenomenon of "gamma pile up" if the size of the neutron generated pulse is not greatly different from the pulse occurring from "gamma pile up." If "gamma pile up" were to result in a noticeable meter reading while the reactor was being restarted within a short time following reactor shutdown, it could mask the true rate of buildup of neutron activity, leading the operator to underestimate the proximity to criticality. Thus, source range detectors must have high sensitivity to neutrons as well as the ability to discriminate between neutron pulses and gamma causes pulses in the presence of a strong background of gamma radiation.
Accordingly, a neutron ex-core start up detector having a high neutron sensitivity and a low gamma sensitivity is desirable. This start up detector additionally should have the ability to withstand very high gamma and neutron fluxes without exhibiting excessive degradation in detector sensitivity.